1. Field of the Invention
The present invention relates to a metal gasket used to seal between opposing surfaces of structural members such as a cylinder block and a cylinder head in an engine and having a bead base plate formed with beads along cylinder bore holes.
2. Description of the Prior Art
Engines in recent years tend to use an aluminum material for structural members of the engine, such as cylinder head and cylinder block, as part of an effort to meet the demands for higher output and lighter weight. The engine structural members made of aluminum, such as cylinder head and cylinder block, though lighter in weight than those made of iron material, have smaller rigidity and thus tend to be displaced relative to each other in large amounts during engine operation. Hence, a metal gasket that seals between the opposing interfaces of these structural members is made of an elastic metal plate formed with beads along and near through-holes corresponding to the cylinder bore holes and water and oil passages.
A conventional metal gasket is known to be constructed of a pair of bead base plates made of an elastic metal plate and formed with beads. When the metal gasket is clamped between the opposing interfaces of the cylinder head and the cylinder block and tightened as by bolts to be fully compressed, the reactionary force produced as the beads of the paired bead base plates are pressed and flattened causes the bead base plates and a stopper plate to be strongly pressed against each other between the opposing interfaces at the beads and in their vicinity, with the beads forming an annular, elastic contact portion, thus producing a sealing action to prevent fluids running through the cylinder bore holes from leaking from the opposing interfaces.
Further, in the case of a cylinder head gasket, during the engine combustion cycle the clearance between the cylinder head and the cylinder block repetitively increases and decreases applying cyclic stresses, i.e., mechanical stresses and thermal stresses, to the metal gasket. The load variation-induced stresses cause large deformations in those portions of the cylinder block and the cylinder head having least rigidity, resulting in the beads permanent set in fatigue or cracking, degrading the sealing performance.
The beads of the metal gasket, when subjected to varying stresses and deformations caused by vibrations between the cylinder block and the cylinder head during engine operation, are apt to result in a fatigue failure. To prevent the fatigue failure, the elastic metal plate has a stopper plate stacked therein which has a stopper function to minimize the deformation of the beads. The stopper plate typically has folded portions to increase its thickness in areas between beads of the bead base plate and the cylinder bore holes. The deformation of the beads is reduced to the extent that the stopper plate thickness has increased. In addition to the stopper function of restricting the deformation of the bead base plate, the stopper plate also has a compensation function to offset a irregularity of clearance between the opposing interfaces.
In the metal gasket described above, simply forming the folded portions in the stopper plate to provide the stopper function results only in the thickness of the folded portions being doubled. Having twice the stopper plate thickness at the folded portions may be more than enough to effect the stopper function and may cause extreme surface pressure concentrations around the cylinder bore holes. The surface pressure concentrations at the folded portions in turn cause an imbalance between the surface pressure around the bore holes and the surface pressure at peripheries (of water and oil holes) other than the bore hole peripheries, degrading the sealing performance around the water and oil holes. Such surface pressure concentrations will also produce stress concentrations and therefore cracks in the folded portions, particularly in bent edge portions defining the bore holes. Further, because the beads of the bead base plate are not given sufficient elastic deformations due to the increased thickness of the folded portions, there will arise a problem of failure to form secure annular contact portions.
Hence, metal gaskets are known (found, for example, in Japanese Patent Laid-Open Nos. 246573/1988 and 26575/1994), in which the deformation of the beads increases to effectively utilize the stopper function of the stopper plate and to prevent excess concentration of the surface pressures around the bore holes when the metal gasket is clamped and tightened between a pair of structural members.
The Japanese Patent Laid-Open No. 246573/1988 discloses a laminated metal gasket having a stopper plate inserted between two bead base plates formed of elastic metal plate, in which a first stopper plate and a second stopper plate thinner that the first stopper plate are stacked together and placed between the two bead base plates and in which the edges of the first stopper plate on the bore hole side are folded over, but spaced from, the second stopper plate to form a compensation portion of a predetermined thickness overlapping the bead base plate. This laminated metal gasket prevents expansion or contraction of a deck surface clearance between the cylinder block and the cylinder head due to combustion gas pressures and the thermal influences of the engine, thus weakening the action of the alternating load on the beads to prevent a permanent set in fatigue.
The Japanese Patent Laid-Open No. 26575/1994 discloses a metal gasket, in which at least one elastic metal plate is formed with beads, and a stopper, which is a separately formed metal plate, is located at a flat portion on the cylinder bore side with respect to the beads and has a thickness corresponding to a clearance dimension around the cylinder bore when tightened. Because the stopper is a separately formed metal plate, the metal gasket has high machining precision of the stopper and can fully adjust the thickness of the stopper, allowing for a uniform surface pressure around the cylinder bore and securing the roundness of the bores to improve the sealing performance.
In the metal gaskets comprising bead base plates made of an elastic metal plate formed with beads around the cylinder bore holes and a stopper plate having a stopper function of minimizing deformation of the beads, there is a possibility that if the level of stopper function performed by the stopper plate can be changed without increasing the number of layers of the stopper plate and the number of parts, a low-cost metal gasket with a simplified construction can be provided while maintaining the sealing performance and the surface pressure balance found in the prior art metal gaskets.